[chore] Move request validation earlier in client (#1531)

This moves checking if the request is valid as early as possible in the
chain. This should ensure that for an invalid request we never bother
acquiring the wait queue and taking up a spot in it.

internal/httpclient/client.go

@@ -153,6 +153,11 @@ func New(cfg Config) *Client {
 // as the standard http.Client{}.Do() implementation except that response body will
 // be wrapped by an io.LimitReader() to limit response body sizes.
 func (c *Client) Do(req *http.Request) (*http.Response, error) {
+	// Ensure this is a valid request
+	if err := ValidateRequest(req); err != nil {
+		return nil, err
+	}
+
 	// Get host's wait queue
 	wait := c.wait(req.Host)
 
@@ -198,11 +203,6 @@ func (c *Client) Do(req *http.Request) (*http.Response, error) {
 		}
 	}
 
-	// Firstly, ensure this is a valid request
-	if err := ValidateRequest(req); err != nil {
-		return nil, err
-	}
-
 	// Perform the HTTP request
 	rsp, err := c.client.Do(req)
 	if err != nil {